Pseudomonas aeruginosa is an opportunistic pathogen which is the most common cause of infection in immunocompromised hosts. This disease has proven to be recalcitrant to the traditional methods used for treatment of bacterial infection. The current approach has been aimed at developing a multivalent vaccine but the immunogens tested so far are toxic, strain specific or both. We propose that a reasonable approach to generating immunity against P. aeruginosa is to find a non-toxic protein that may or may not be associated with virulence, that is present at the cell surface or that might be concentrated at the site of infection. One group of extracellular proteins that have not been investigated as possible protective immunogens are the pyocins. Pyocins are bacteriocins secreted by certain strains of Pseudomonas aeruginosa which inhibit the growth of certain other strains of the same organism. They are proteins and are produced and secreted during normal growth. Our preliminary studies indicate that a particular pyocin, S2, is produced during an actual infection by P. aeruginosa and the host responds by producing specific antibody against S2. Pyocin S2 is non-toxic and passive immunization experiments we have just recently completed with rabbit anti-S2 antisera in the mouse burn wound sepsis model suggest that indeed anti-S2 antibody may offer some protection. The present proposal is aimed at: 1) establishing by immunofluorescent microscopy and indirect ELISA whether or not pyocin is continuously present at the outer surface of the cell; 2) determining the prevalence of pyocin S2 in clinical isolates of P. aeruginosa by immunofluorescent microscopy or by testing for biological activity; this is important in order to establish the potential of S2 as a component of a multivalent vaccine against Pseudomonas infection; and 3) more extensively establishing the extent of protection afforded by anti-S2 antisera through passive immunization studies in mice.